1. Field of the Invention
The invention relates to the use of dams on printed circuit boards (PCB) in, for example, PCMCIA PC cards, single line memory modules (SIMMs), and dual-in-line memory modules (DIMMs), as well as multi-chip modules (MCMs) in general. The dams may act as both a restraint or container for a liquid or gel encapsulant for chips, and as a standoff to support the card's shell to prevent the shell from contacting the chips or other portions of the PCB.
2. State of the Art
The Personal Computer Memory Card International Association (PCMCIA) defines standards for "credit card" sized "PC cards." The PC cards are widely used and typically perform memory and/or input/output (I/O) functions. PCMCIA standard PC cards have a standard length and width of 85.6 mm (3.370") and 54.0 mm (2.126"). However, the thickness varies depending on the "types" of card. Type 1 has a thickness of 3.3 mm (0.130"); Type 2 has a thickness of 5.0 mm (0.197"); and Type 3 has a thickness of 10.5 mm (0.413").
Referring to FIGS. 1A and 1B, a PC card 10 includes a card shell 14 and a built-in connector 18. Shell 14 is formed as an upper surface or section 22A and a lower surface or section 22B that are glued, welded, or otherwise affixed to each other, along an edge 24. Upper section 22A includes an extended planar portion 26A and lower section 22B includes an extended planar portion 26B.
Referring to FIG. 2A, in operation, PC card 10 is connected through connector 18 to a socket 30. PC cards are commonly used with portable computers (such as laptop, notebook, and sub-notebook computers), and in connecting various peripherals to a desk-top computer. In FIG. 2A, dashed lines represent a host system 34 (which could be a portable computer or peripheral interface) into which PC card 10 may be inserted. In FIG. 2A, PC card 10 is shown with an optional RJ-11 plug, which is used in connection with many PC cards.
Socket 30 is connected to a PCMCIA host bus adapter 38 through a PCMCIA socket interface 40. Host bus adapter 38 may be controlled in part through enabling software 44 (such as configuration and event notification software and run-time software) that is outside the scope of the present invention. Data passes between host bus adapter 38 and a host system controller 48 through a host bus 50. Host system controller 48 typical contains a microprocessor and various other electronic components.
Referring to FIG. 2B, host system 34 may be connected to a peripheral device 36, such as a printer or remote computer (which may contain its own PC card).
In FIG. 3, upper section 22A is separated from lower section 22B to illustrate the internal structure of PC card 10. Referring to FIG. 3, a printed circuit board (PCB) 54 is positioned in a "substrate area" 56 of lower section 22B. Connector 18, positioned in an "interconnect area" 60 of lower section 22B, communicates with components on PCB 54 through a bus 64. For purposes of illustration, chips 68, 70, 72, 74, and 76 are shown on PCB 54, although there may be many more chips or other components.
Upper and lower sections 22A and 22B are somewhat rigid. However, when a sufficient force is applied, upper and/or lower sections 22A and 22B, and particularly portions 26A and 26B, will flex or compress toward PCB 54. In such an event, there is some significant chance that upper and/or lower portions 26A and 26B will short out or otherwise damage components carried on PCB 54. Shorting out or otherwise damaging components is, of course, undesirable.
Typically, an electrical "chip" includes a semiconductor die that is wire bonded or otherwise electrically connected to a circuit board or lead frame and an encapsulant that surrounds and protects the die and wires. Encapsulants have been made of plastic in various forms, including transfer-molded packages as well as glob top or gel masses.
Filled epoxy resins are commonly used polymeric materials for lead frame-based device encapsulation by transfer molding. The molded encapsulant protects the die and wires and inner lead fingers from unfavorable mechanical, chemical, electrical, and thermal environments. Reasons for encapsulation include to insulate and isolate adjacent conductors (bond wires) from each other, to improve the vibration and shock resistance of the assembly, to provide mechanical rigidity to the assembly, to provide protection from atmospheric pollution, and to shrink the device size below that obtainable with preformed ceramic or metal packages. Because of their good dimensional tolerances, plastic packages are suitable for mechanized assembly. A deficiency of such polymer-encapsulated chips is that water will readily permeate even hydrophobic polymers. Therefore, a significant reliability issue in using plastic encapsulated chips is moisture-induced failures, including metallization corrosion. See Electronic Materials Handbook, Vol. 1 Packaging, ASM International, 1989, p. 802-03; Microelectronics Packaging Handbook, Edited R. Tummala et al., 1989, Van Nostrand Reinhold, pp. 760-61.
Merely as an example, the construction of a plastic-encapsulated DIP is now described. First, the semiconductor die is attached to the lead frame. Second, wirebonding is performed between the bond pads on the die and the corresponding leads. Third, transfer molding is used for plastic encapsulation. Fourth, the leads are trimmed, formed and solder coated. The die can sometimes be conformally coated with special polymeric substances prior to the transfer molding to impart additional protection from external moisture and from stresses caused by the molding operation and protection from thermal contraction mismatches. See Microelectronics Packaging Handbook, Edited R. Tummala et al., 1989, Van Nostrand Reinhold, pp. 760-61.
A glob top encapsulant refers to another type of polymer-encapsulated "package" in which the chip is attached to a conventional laminated, ceramic or silicon substrate with the polymeric encapsulant dispensed over it as a "glob top." These packages may be additionally enclosed under a cover sealed with polymer. A glob top encapsulant is usually a highly viscous liquid or paste that encapsulates either a microelectronic device directly mounted on a printed wiring board or a hybrid assembled on a substrate (ceramic board). Glob tops may be filled thermoset plastics. They are based primarily on epoxy or silicone technology and contain an inorganic filler or fillers. To reduce entrapment of air and to improve uniformity of compositions, a glob top encapsulant that generally comprises both resin and hardener parts may be prepared under reduced pressure with high shear mixing.
A desirable glob top encapsulant may desirably possess all of the following characteristics: (1) balanced viscosity, thixotropy and resistance to bleed at typical application and curing temperatures; (2) quick curing without void formation; (3) a low level of potentially leachable ionic contaminants; (4) mechanical strength and low-stress performance with an appropriate glass transition temperature; (5) moisture resistance and hydrophobicity; (6) compatibility with and proper adhesion to adjacent objects including substrate board, chip surface, and electroconductive circuitry; (7) desirable heat dissipation thermal conductivity; (8) batch-to-batch consistency of performance and quality; and (9) good electrical insulative properties. See Electronic Materials Handbook, Vol. 1 Packaging, ASM International, 1989, p. 803; Microelectronics Packaging Handbook, Edited YR. Tummala et al., 1989, Van Nostrand Reinhold, p. 761.
Unconstrained glob top encapsulants are commonly employed in the art. That is to say, the encapsulant is literally applied as a "glob" to the die and permitted to expand laterally until surface tension and viscosity naturally limit its spread under gravity. Such an approach may be wasteful of material, and may result in board areas being coated which are intended to be left clear. Furthermore, use of an unconstrained glob top requires a greater mass of encapsulant to ensure coating of the wire bonds and die to an adequate depth, the latter being somewhat difficult to predict. Furthermore, the use of unconstrained encapsulants limits their use to fairly thick or viscous materials, which may include undesirable voids from trapped air (notwithstanding the use of high shear mixing techniques as described above), and to materials which actually dry, set or cure to a stable-dimensioned mass. Thus, the use of non-curing gels, which may be desirable in some circumstances for ease of failure analysis and rework, is excluded.
Dam structures have been developed to limit lateral encapsulant spread. See U.S. Pat. No. 5,436,203 to Lin. However, Lin's dam structures provide little; if any, physical protection from adverse contact by other package elements, and are intended to be employed solely with encapsulants which set or dry to a dimensionally stable mass.